The field of the invention is magnetic resonance imaging systems, such as those used for medical diagnosis; and more particularly, to coils employed in such systems to establish magnetic field gradients within a region of interest.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but randomly oriented magnetic components in the perpendicular, or transverse, plane (X-Y plane) cancel one another. If the substance or tissue is subjected to a magnetic field (excitation field B.sub.1) which is in the X-Y plane and which is near the Larmor frequency, the net aligned moment (M.sub.z) may be rotated, or "tipped", into the X-Y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the X-Y plane at the Larmor frequency. The practical value of this phenomenon resides in the electrical signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The region of interest may be a small portion of a patient's anatomy, such as the head or heart, or a much larger portion, such as the entire thorax. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective X, Y and Z axes. The magnetic field gradients are produced by a trio of coil assemblies placed around the object being imaged. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
In order to accommodate the imaging of large portions of a patient, each gradient field coil assembly must produce a magnet field that varies linearly along one axis of a very large volume. This requirement necessitates a substantial amount of power to drive the coils and the ability to vary the field at relatively high speed. Even when the region of interest is relatively small, the power consumption of the conventional gradient coils remains the same as for larger regions.
Patients being imaged can experience peripheral nerve stimulation, if the amplitude and temporal rate of change of the gradient fields are great enough. For example, during imaging of the thorax, the head of the patient is positioned in the fringe region of the gradient field where it is exposed to a cross-term of the field. This exposure can produce small electric currents in the patient's head which stimulate the nerves, in the nose for example, and producing an annoying sensation. Although not life threatening, this phenomenon can be considered invasive and desirable to avoid.